1. Validation of EM Part of Geant4
February 22, Geant4 Work Shop
Tsuneyoshi Kamae/Tsunefumi Mizuno

2. Purpose and Plan of this Talk
We have validated EM processes in Geant4 important for gamma-ray satellite GLAST. Energy range of GLAST is 20 MeV GeV. So we need to validate physics processed down to ~10 MeV and lower.
Particle Ionization Energy Loss in Matters
Bethe-Bloch Formula (pp. 3-5)
Landau Distribution (pp. 3-5)
Range (p. 6)
Processed related to electromagnetic shower
Pair creation (pp. 8-9)
Bremsstrahlung (p. 10)
Moller scattering/BhaBha scattering (pp )
Comparison of shower profile with experimental data (pp )
Comparison of shower profile with EGS4 (pp )
Tracker (Si, Pb)
Calorimeter (CsI)
We use Geant4 2.0

3. Ionization Energy loss in matters
Shoot proton/electron in Pb and Si.
Cutoff Length:
0.4mm(e-), 0.1mm(others)
Thin absorber:
Landau distribution
Thick absorber:
Bethe-Bloch formula
Check mean energy loss (thick absorber), shape and most probable energy loss (thin absorber).

4. Bethe-Bloch formula for protons
Geant4 well reproduces ionization energy loss in thick Si and Pb of proton down to 10 MeV

5. Landau Distribution for protons and electrons
Fluctuation of energy loss and the most probable value in thin Si and Pb are appropriate.

6. Range of protons Shoot protons of 200 MeV and 1 GeV in CsI, Pb and W
Compare the results with NIST data: (
Show good agreement within 0.6%.

7. Processes related to EM shower
Pair creation (pp. 8-9)
(and compton scattering and photo-electric effect)
Bremsstrahlung (p. 10)
Moller scattering (pp )
Bhabha scattering (pp )
We need to validate these processes, especially in low-energy, since
Pair creation and EM shower are key processes to determine gamma-ray energy and direction in the GLAST.
Low energy electrons may suffer large-angle scattering and cause trigger in the GLAST.
Cutoff
0.04mm(e-)
0.01mm(others)
Tracker
(Si-Strip detectors)
Calorimeter
(CsI scintillator) e-

8. Pair creation – cross section
Compare the cross section in Pb calculated by Geant4 with that of a reference (
Geant4 correctly calculates cross section down to ~100 keV

9. Pair creation-angular distribution of emitted electrons
e- 4 MeV
Events/Bin
Shoot 20 MeV gammas into Pb absorber (10% RL)
Compare the angular distribution of generated e- with that of theoretical formula
0.125
0.25
0.375
0.5
Theta [rad]
e- 18 MeV
Events/Bin
We found discrepancies for low energy electrons and fixed the code
0.125
0.25
0.375
0.5
Theta [rad]

10. Bremsstrahlung -- angular distribution of generated photons
Shoot 20 MeV e- into Pb absorber (10% RL)
Compare the angular distribution of generated photons with that of theoretical formula
gamma 2 MeV
gamma 6 MeV
Geant4 well reproduces the angular distribution of photons generated via bremsstrahlung down to ~1 MeV
gamma 10 MeV

11. Moller/Bhabha scattering (scattering with e-/e+)
Shoot e- of 20 MeV and 100 MeV into Pb absorber (1% RL)
Compare the angular distribution of scattered electrons with that of theoretical formula

12. Moller scattering (with e-)
Bhabha scattering (with e+)
Theoretical furmula
Geant4
20 MeV
20 MeV
100 MeV
100 MeV
G4 well reproduces the theoretical formula down to 20 MeV

13. EM shower profile – Crannell experiment (1)
Cranell experimtns:
Carol Jo Crannell, et al., Phys. Rev. 184 (1969) 426
shoot 1GeV e- into water tank (10 rings and 11 layers)
Compare the G4 results with that of experimental data and EGS4 (Professor Nelthon and Liu).

14. EM shower profile – Crannell experiment (2)
longitudinal profile
lateral profiles
Shower profile of G4 is narrower than that of experiment and EGS4

15. EM shower in Pb layers (1)
To investigate the EM shower profile in the GLAST Tracker, we constructed 10 Pb layers(0.1RL each) with air (3cm). The radius is 0.2 Moliere radius (core of shower)
Geant4
EGS4
20 MeV, longitudinal profile Pb
lateral profiles
air

16. EM shower in Pb and air (2)
Geant4
Geant4
EGS4
EGS4
500 MeV, longitudinal profile
5 GeV, longitudinal profile
lateral profiles
lateral profiles
Energy deposition is higher in Geant4 (low energy region)
Geant4 shows narrower shower profile (and could not be solved by fixing the angular distribution of pair creation)

17. Effect of cutoff energy (length)
0.4mm, 0.04mm (e-), 0.1mm, 0.01mm(others)
The discrepancy can not be attributed to cutoff length (energy)
Geant4
EGS4
The reason of the narrower shower profile is unrevealed.

18. Summary We have validated EM part of Geant4 (2.0).
Energy loss in matters is well simulated down to ~10 MeV.
Angular distribution of pair-created e- was found to be inappropriate and fixed.
Cross section of pair creation, angular distribution of bremsstrahlung/Moller scattering/Bhabha scattering were validated down to ~10 MeV.
Shower profile of G4 is slightly narrower than that of experimental data and EGS4. The reason of this is unknown.